State-of-the-art methods for 3D reconstruction of faces from a single image require 2D-3D pairs of ground-truth data for supervision. Such data is costly to acquire, and most datasets available in the literature are restricted to pairs for which the input 2D images depict faces in a near fronto-parallel pose. Therefore, many data-driven methods for single-image 3D facial reconstruction perform poorly on profile and near-profile faces. We propose a method to improve the performance of single-image 3D facial reconstruction networks by utilizing the network to synthesize its own training data for fine-tuning, comprising: (i) single-image 3D reconstruction of faces in near-frontal images without ground-truth 3D shape; (ii) application of a rigid-body transformation to the reconstructed face model; (iii) rendering of the face model from new viewpoints; and (iv) use of the rendered image and corresponding 3D reconstruction as additional data for supervised fine-tuning. The new 2D-3D pairs thus produced have the same high-quality observed for near fronto-parallel reconstructions, thereby nudging the network towards more uniform performance as a function of the viewing angle of input faces. Application of the proposed technique to the fine-tuning of a state-of-the-art single-image 3D-reconstruction network for faces demonstrates the usefulness of the method, with particularly significant gains for profile or near-profile views.

Assessing the severity of liver fibrosis has direct clinical implications for patient diagnosis and treatment. Liver biopsy, typically considered the gold standard, has limited clinical utility due to its invasiveness. Therefore, several imaging-based techniques for staging liver fibrosis have emerged, such as magnetic resonance elastography (MRE) and ultrasound elastography (USE), but they face challenges that include limited availability, high cost, poor patient compliance, low repeatability, and inaccuracy. Computed tomography (CT) can address many of these limitations, but is still hampered by inaccuracy in the presence of confounding factors, such as liver fat. Dual-energy CT (DECT), with its ability to discriminate between different tissue types, may offer a viable alternative to these methods. By combining the "multi-material decomposition" (MMD) algorithm with a biologically driven hypothesis we developed a method for assessing liver fibrosis from DECT images. On a twelve-patient cohort the method produced quantitative maps showing the spatial distribution of liver fibrosis, as well as a fibrosis score for each patient with statistically significant correlation with the severity of fibrosis across a wide range of disease severities. A preliminary comparison of the proposed algorithm against MRE showed good agreement between the two methods. Finally, the application of the algorithm to longitudinal DECT scans of the cohort produced highly repeatable results. We conclude that our algorithm can successfully stratify patients with liver fibrosis and can serve to supplement and augment current clinical practice and the role of DECT imaging in staging liver fibrosis.

The ability of dual-energy computed-tomographic (CT) systems to determine the concentration of constituent materials in a mixture, known as material decomposition, is the basis for many of dual-energy CT's clinical applications. However, the complex composition of tissues and organs in the human body poses a challenge for many material decomposition methods, which assume the presence of only two, or at most three, materials in the mixture. We developed a flexible, model-based method that extends dual-energy CT's core material decomposition capability to handle more complex situations, in which it is necessary to disambiguate among and quantify the concentration of a larger number of materials. The proposed method, named multi-material decomposition (MMD), was used to develop two image analysis algorithms. The first was virtual unenhancement (VUE), which digitally removes the effect of contrast agents from contrast-enhanced dual-energy CT exams. VUE has the ability to reduce patient dose and improve clinical workflow, and can be used in a number of clinical applications such as CT urography and CT angiography. The second algorithm developed was liver-fat quantification (LFQ), which accurately quantifies the fat concentration in the liver from dual-energy CT exams. LFQ can form the basis of a clinical application targeting the diagnosis and treatment of fatty liver disease. Using image data collected from a cohort consisting of 50 patients and from phantoms, the application of MMD to VUE and LFQ yielded quantitatively accurate results when compared against gold standards. Furthermore, consistent results were obtained across all phases of imaging (contrast-free and contrast-enhanced). This is of particular importance since most clinical protocols for abdominal imaging with CT call for multi-phase imaging. We conclude that MMD can successfully form the basis of a number of dual-energy CT image analysis algorithms, and has the potential to improve the clinical utility of dual-energy CT in disease management.

The use of image-based automatic defect recognition (ADR) systems in a production line often requires strict processing-time specifications. On the other hand, the typical high-performance requirement of such system calls for the use of sophisticated, computationally-complex algorithms. Addressing the conflicting requirements of fast throughput and high detection performance is a significant challenge. In this paper we present a 3D learning-based ADR approach for industrial parts. The proposed method first extracts defect candidate regions using morphological closing and template matching. Then a local registration-based approach is utilized to produce accurate defect segmentation mask. Finally, 29 features including geometric features and texture features derived from grey level co-occurrence matrix are calculated for each candidate region, and a fast random forests classifier is used to classify the candidate regions as defect or defect-free. This approach was developed into a fully automated system for detecting casting defects in aluminum industrial parts depicted in 3D Computed Tomographic (CT) images. The system was tested on 31 images with 49 cavities and porosities defects, achieving a sensitivity of 94% with an average 3.5 false detections per part.

In machine vision applications it is often desirable to make metric measurements with a single camera. However, operational constraints can limit the use of off-line calibration methods and the lack of reference planes can prevent the use of current single-view metrology techniques. In this paper, we describe the design, implementation, and validation of a visual inspection system for the measurement of angles from a single, uncalibrated camera. We provide a step-by-step, single-view metrology approach which we demonstrate in an operational setting. We first assess the imaging conditions present using hypothesis testing based on a norm-induced metric between possible world to image mappings. Our metric provides a measure of localization error derived from physical measures in the world. The hypothesis testing demonstrates that affine viewing conditions are present in our setting and we show how the affine approximation can be used to make measurements of points off the plane of interest. We employ our algorithm in a manufacturing plant to make blade angle measurements of variable stator vanes in jet engines.

It has been shown that under a small number of assumptions, observations of people can be used to obtain metric calibration information of a camera, which is particularly useful for surveillance applications. However, previous work had to exclude the common criticial configuration of the camera’s principal point falling on the horizon line and very long focal lengths, both of which occur commonly in practise. Due to noise, the quality of the calibration quickly degrades at and in the vicinity of these configurations. This paper provides a robust solution to this problem by incorporating information about the motion of people into the estimation process. It is shown that under the assumption that people walk with a constant velocity, calibration performance can be improved significantly. In addition to solving the above problem, the incorporation of temporal data also helps to take correlations between subsequent detections into consideration, which leads to an up-front reduction of the noise in the measurements and an overall improvement in auto-calibration performance. 1

In the context of visual surveillance of human activity knowledge about a camera's internal and external parameters is useful, as it allows for the establishment of a connection between image and world measurements. Unfortunately, calibration information is rarely available and difficult to obtain after a surveillance system has been installed. In this paper, a method for camera autocalibration based on information gathered by tracking people is developed. It brings two main contributions: first, we show how a foot-to-head plane homology can be used to obtain the calibration parameters and then we show an approach how to efficiently estimate initial parameter estimates from measurements; second, we present a Bayesian solution to the calibration problem that can elegantly handle measurement uncertainties, outliers, as well as prior information. It is shown how the full posterior distribution of calibration parameters given the measurements can be estimated, which allows making statements about the accuracy of both the calibration parameters and the measurements involving them

This paper proposes a method for three-dimensional reconstruction of surfaces that takes advantage of the symmetry resulting from alternating the positions of a camera and a light source. This set up allows for use of the Helmholtz reciprocity principle to recover the shape of smooth surfaces with arbitrary bidirectional reflectance distribution functions without requiring the presence of texture, as well as for exploiting mutual occlusions between images. Different from previous methods, the technique works with as few as one reciprocal pair, and recovers surface depth and orientation simultaneously by finding the global minimum of an error function via dynamic programming. Since the error is a function not just of depth but of surface orientation as well, the reconstruction is subject to tighter geometric constraints. Given a current estimate of surface geometry and intensity measurements in one image, Helmholtz reciprocity is used to predict the pixel intensity values of the second image. The dynamic program finds the reconstruction that minimizes the total difference between the predicted and measured intensity values. This approach allows for the reconstruction of surfaces displaying specularities and regions of high curvature, which is a challenge commonly encountered in the optical inspection of industrial parts. Results with real data show the quality of the reconstruction obtained with the proposed algorithm.

This paper addresses the problem of calibrating a pinhole camera from images of a surface of revolution. Camera calibration is the process of determining the intrinsic or internal parameters (i.e., aspect ratio, focal length, and principal point) of a camera, and it is important for both motion estimation and metric reconstruction of 3D models. In this paper, a novel and simple calibration technique is introduced, which is based on exploiting the symmetry of images of surfaces of revolution. Traditional techniques for camera calibration involve taking images of some precisely machined calibration pattern (such as a calibration grid). The use of surfaces of revolution, which are commonly found in daily life (e.g., bowls and vases), makes the process easier as a result of the reduced cost and increased accessibility of the calibration objects. In this paper, it is shown that two images of a surface of revolution will provide enough information for determining the aspect ratio, focal length, and principal point of a camera with fixed intrinsic parameters. The algorithms presented in this paper have been implemented and tested with both synthetic and real data. Experimental results show that the camera calibration method presented is both practical and accurate.

This paper addresses the problem of recovering the 3D shape of a surface of revolution from a single uncalibrated perspective view.The algorithm introduced here makes use of the invariant properties of a surface of revolution and its silhouette to locate the image of the revolution axis, and to calibrate the focal length of the camera.The image is then normalized and rectified such that the resulting silhouette exhibits bilateral symmetry.Such a rectification leads to a simpler differential analysis of the silhouette, and yields a simple equation for depth recovery.Ambiguities in the reconstruction are analyzed and experimental results on real images are presented, which demonstrate the quality of the reconstruction.

This paper presents a novel method for hand tracking. It uses a 3D model built from quadrics which approximates the anatomy of a human hand. This approach allows for the use of results from projective geometry that yield an elegant technique to generate the projection of the model as a set of conics, as well as providing an efficient ray tracing algorithm to handle self-occlusion. Once the model is projected, an Unscented Kalman Filter is used to update its pose in order to minimise the geometric error between the model projection and a video sequence on the background. Results from experiments with real data show the accuracy of the technique. 1

This paper presents a practical technique for model-based 3D hand tracking. An anatomically accurate hand model is built from truncated quadrics. This allows for the generation of 2D profiles of the model using elegant tools from projective geometry, and for an efficient method to handle self-occlusion. The pose of the hand model is estimated with an Unscented Kalman filter (UKF), which minimizes the geometric error between the profiles and edges extracted from the images. The use of the UKF permits higher frame rates than more sophisticated estimation methods such as particle filtering, whilst providing higher accuracy than the extended Kalman filter The system is easily scalable from single to multiple views, and from rigid to articulated models. First experiments on real data using one and two cameras demonstrate the quality of the proposed method for tracking a 7 DOF hand model.

Addresses the problem of motion estimation from profiles (apparent contours) of an object rotating on a turntable in front of a single camera. A practical and accurate technique for solving this problem from profiles alone is developed. It is precise enough to reconstruct the shape of the object. No correspondences between points or lines are necessary. Symmetry of the surface of revolution swept out by the rotating object is exploited to obtain the image of the rotation axis and the homography relating epipolar lines in two views robustly and elegantly. These, together with geometric constraints for images of rotating objects, are used to obtain first the image of the horizon, which is the projection of the plane that contains the camera centers, and then the epipoles, thus fully determining the epipolar geometry of the image sequence. The estimation of this geometry by this sequential approach avoids many of the problems found in other algorithms. The search for the epipoles, by far the most critical step, is carried out as a simple 1D optimization. Parameter initialization is trivial and completely automatic at all stages. After the estimation of the epipolar geometry, the Euclidean motion is recovered using the fixed intrinsic parameters of the camera obtained either from a calibration grid or from self-calibration techniques. Finally, the spinning object is reconstructed from its profiles using the motion estimated in the previous stage. Results from real data are presented, demonstrating the efficiency and usefulness of the proposed methods.

In this paper we address the problem of recovering structure and motion from the contours of a smooth-curved surface. A novel and simpler technique for computing the structure of an object from its profiles is introduced. Experiments with real data show encouraging results, which are comparable to those obtained from much more sophisticated techniques. Furthermore, a new method for motion estimation from sequences of profiles is proposed. Preliminary results demonstrate the feasibility of the algorithm. 1 Introduction The recovering of structure and motion from sequences of images is a central problem in computer vision, and its solution has generated a rich pool of algorithms [8, 1]. Most of these algorithms rely on correspondences of points or lines between images, and work well when the scene being viewed is composed of polyhedral parts. However, for smooth surfaces without noticeable texture, point and line correspondences may not be easily established. In this case the profile of...

This paper addresses the problem of estimating the epipolar geometry from apparent contours in two special cases: under weak perspective and for circular motion. An appropriate parametrization of the fundamental matrix is introduced for both cases, as well as suitable cost functions for the estimation of the epipoles. The algorithm used in the affine approximation proven to be robust and accurate under several conditions. The circular motion case turned out to be much more difficult, but for a wide baseline the method introduced here is successful. For small viewing angles the technique is too sensitive to noise to be used in practice. Nevertheless, circular motion with small baseline can be well modeled by an affine camera system, and this approximation should be used in this circumstance.

This paper introduces an extension of Hartley's self-calibration technique based on properties of the essential matrix, allowing for the stable computation of varying focal lengths and principal point. It is well known that the three singular values of an essential must satisfy two conditions: one of them must be zero and the other two must be identical. An essential matrix is obtained from the fundamental matrix by a transformation involving the intrinsic parameters of the pair of cameras associated with the two views. Thus, constraints on the essential matrix can be translated into constraints on the intrinsic parameters of the pair of cameras. This allows for a search in the space of intrinsic parameters of the cameras in order to minimize a cost function related to the constraints. This approach is shown to be simpler than other methods, with comparable accuracy in the results. Another advantage of the technique is that it does not require as input a consistent set of weakly calibrated camera matrices (as defined by Harley) for the whole image sequence, i.e. a set of cameras consistent with the correspondences and known up to a projective transformation.

This paper investigates the trifocal tensor for an affine trinocular rig and defines an affine trifocal tensor.The question of the degrees of freedom of the tensor entries will be addressed, and a novel algorithm to compute the tensor by a linear technique is presented.It will be shown that 4 point or 8 line correspondences along the three images are enough for a reliable computation of the trifocal tensor in the affine case (uncalibrated, weak perspective camera model), in contrast to the projective case, where at least 7 point or 13 line correspondences are needed for a linear computation (and usually with poor results).An analysis of the error in the approximation of a generic trifocal tensor by the affine trifocal tensor is carried out and preliminary experiments with synthetic and real data show the reliability and robustness of the approximation under a wide range of conditions.